Polypropylenes are ideally suited to the preparation of coatings and insulating materials designed for use at operating temperatures in excess of those that can be withstood by other polyolefins such as, for example, polyethylene, which exhibits lower softening and melting temperatures. Polyethylenes have a maximum melting point, as measured by differential scanning calorimetry (DSC), of about 135° C., whereas polypropylenes can have melting points as high as 175° C. As such, polypropylenes can withstand higher operating temperatures without being permanently damaged or deformed.
Other attractive features of polypropylenes are their high rigidity, abrasion resistance, impact resistance, toughness, low cost and relatively low density. Applications for polypropylene-based coatings and insulations include insulation for electrical wires and cables, heat-shrinkable, corrosion protective sleeves for high-temperature transmission pipeline joints, heat-shrinkable tubing or shapes for electrical insulation and mechanical protection, or other applications requiring greater toughness and rigidity than is afforded by polyethylene-based systems.
In order to maximize heat resistance and physical properties, it is necessary to impart some thermoset characteristic to the material. This is done by crosslinking the polymer to some required degree. This renders the material resistant to melting and flowing when it is heated to a temperature close to or above the crystalline melting point of the highest melting point polymer component of the composition. This characteristic is necessary for the production of high temperature insulation materials and heat-shrinkable articles. In the latter case crosslinking imparts controlled shrinkage characteristics and renders the material resistant to melting when it is heated to the temperature necessary to effect shrinkage.
Several examples of crosslinked, predominantly polypropylene-based polymer compositions are known in the prior art. U.S. Pat. No. 6,569,915, which is incorporated herein by reference in its entirety, describes heat-shrinkable articles which comprise a radiation-crosslinked composition based predominantly on polypropylene and also comprising an ethylene-propylene elastomer. U.S. Pat. Nos. 6,455,637, 6,465,547 and 6,794,453, incorporated herein by reference in their entirety, relate to articles comprised of moisture-crosslinked, predominantly polypropylene based compositions which also include amounts of silane-grafted polyolefin. The predominantly polypropylene-based compositions disclosed and claimed in these patents all have high rigidity, toughness and heat deformation resistance, making them suitable for use in demanding applications such as protective sleeves for high-temperature transmission pipelines where the operating temperature is typically in the range of 110-130° C., or above.
There are, however, situations where it is desirable to provide compositions and articles having rigidity, toughness and mechanical integrity similar to that of crosslinked polypropylene-based compositions, but with moderated high-temperature capability as determined by those mechanical properties dependent upon the performance of the constituent polymers above their softening points. However, these compositions should provide similar long-term thermal stability with respect to useful service life at the intended operating temperature. In addition, there is a need for compositions which economically meet these requirements and offer improved manufacturing processibility.